[gnome-builder] cargo: add comment regarding path renaming

[Christian Hergert <chergert src gnome org>]

commit 7c9630cc3dc8eec9d598cf1ac0f8e3dc71402641
Author: Christian Hergert <chergert redhat com>
Date:   Mon Feb 18 12:02:06 2019 -0800

    cargo: add comment regarding path renaming

 src/plugins/cargo/cargo_plugin.py | 2 ++
 1 file changed, 2 insertions(+)
---
diff --git a/src/plugins/cargo/cargo_plugin.py b/src/plugins/cargo/cargo_plugin.py
index ea2b6e5e9..f1140654c 100644
--- a/src/plugins/cargo/cargo_plugin.py
+++ b/src/plugins/cargo/cargo_plugin.py
@@ -133,6 +133,8 @@ class CargoPipelineAddin(Ide.Object, Ide.PipelineAddin):
         build_launcher.push_argv('--')
         build_launcher.push_argv('--error-format')
         build_launcher.push_argv('short')
+
+        # Format is --remap-path-prefix from=to
         build_launcher.push_argv('--remap-path-prefix')
         build_launcher.push_argv('=' + pipeline.get_srcdir())